1. Field of the Invention
The present invention relates to method and apparatus for displaying grayscale images, and more particularly, to a method and apparatus for displaying grayscale images on a monochrome LCD screen.
2. Description of Prior Art
LCD (liquid crystal display) is nowadays in the information age widely used on small-scale data processing units which can be handheld or portable as PDAs (Personal Digital Assistant), notebook computers, subnotebook computers, electronic dictionaries, and the like. A monochrome LCD screen is composed of an array of tiny LCD cells, each capable of displaying only two colors: white and black. Presently, small-scale data processing units are primarily used for word processing purposes so that the LCD only needs to display text or icons. A monochrome screen is thus adequate to support most of the applications on a small-scale data processing unit.
However, with the coming of the multimedia applications, there are demands for the small-scale data processing units to display images. Responding to such demands, there are offered grayscale and color displays. These displays can show grayscale or color images in high resolution but, however, are often expensive to implement. Moreover, these displays often require customized drivers so that cost is high.
On PDAs and electronic dictionaries, a cost-effective method for displaying (simulating) grayscale images on a monochrome screen is to divide a grayscale image into multiple pages of "black and white" images and then display these pages in a predetermined sequence cyclically at a very high rate on the screen. One example of such a method is depicted in FIG. 1. In the example shown, the monochrome LCD screen is used to simulate a 4-level grayscale display. The four gray levels are as listed below:
______________________________________ Gray Level Color ______________________________________ 0 Black 1 Dark Gray 2 Light Gray 3 White ______________________________________
Using the method, an original grayscale image is divided into three pages. If a pixel on the original image has a gray level of 0 (black), the corresponding dots on the three pages are all black; if gray level 1 (dark gray), the corresponding dots on thee first page and the second page are black and that on the third page is white; if gray level 2 (light gray), the corresponding dot on the first page is black and that on the second page and third page are white; and if gray level 3 (white), the corresponding dots on all the three pages are white.
To simulate the grayscale color of a pixel, the three pages are displayed cyclically at a very fast rate in a predetermined sequence from Page 1 through Page 3. In hardware implementation, an LCD cell displays white when a voltage V.sub.rm0 is applied thereto, and displays black when a voltage V.sub.rm3 is applied thereto. Accordingly, to make an LCD cell simulate a gray level 0 color (black), the voltage sequence (V.sub.rm3, V.sub.rm3, V.sub.rm3) is applied cyclically to the LCD cell; to simulate a gray level 1 color (light gray), the voltage sequence (V.sub.rm3, V.sub.rm0, V.sub.rm0) is applied; to simulate a gray level 2 color (dark gray), the voltage sequence (V.sub.rm3, V.sub.rm3, V.sub.rm0) is applied; and to simulate a gray level 3 color (white), the voltage sequence (V.sub.rm0, V.sub.rm0, V.sub.rm0) is applied.
The aforementioned method, however, has a drawback in that blinking of the simulated grayscale image is obvious to the user's eyes. The persistence of vision in the human eye cause the user to perceive the change of dots from black to white or from white to black when the frequency of such a change is low. It can be seen from FIG. 1 that in the aforementioned method the spatial frequency of the layout of the dots is low, allowing the user's eyes to perceive the changes of dots between black and white.
Besides, the period of the voltage sequence applied is too long, so that the frequency of change is low. Assume that the period required to display one page is T, since a total of (N-1) pages are required to simulate an N-level grayscale image, the time needed to display a complete cycle of all the (N-1) pages (call refresh time) is (N-1) * T The rate of tonal change in the simulated grayscale image is thus: EQU F.sub.t1 =1/(N-1)*T!
where f=1/T.
The refresh frequency must be high enough to eliminate the blinking effect of the simulated grayscale image to the user's eyes.